when you connect via POP you use an SMTP server to inject messages into the server.
you need to allow him to authenticate on the SMTP server and allow relay for authenticated users

since he is from an outside domain he cannot relay by default like the users that are local to the machine. you also need to set his client to the " my outgoing server requires authentication" setting.
the reason you can send messages to local (inside the domain) is the SMTP server accepts and relays messages for its own domain but not for strange ppl (ie outside domains).

Pleasure
Well that depends on how his program picks up the mail information. if it monitors his Pc`s folders your telnet wont be logged.
unless he has admin access to the server i dont see how he can monitor it other wise.

And remember that POP is only the incoming protocol. with only POP you wont be able to send email. you need the SMTP server to send email to anyone.

To make telnet work you will have to either authenticate your SMTP session via the command line or add your IP to a allow relay form list in the SMTP server settings..

I don't have access to the Exchange server, and don't intend changing it. Just trying to get a feel for how the system works. I've never touched Exchange in my life.

I have established that the guy is solely using his external account to recieve and send. It's stored on an external server, which he then connects to with his mail app', using POP3 and SMTP. It's this server that is monitoring his mail, and to which he can view logs of.

I have been sending a few test mails through Telnet and I think I've sunk my theory...

My feeling was that I was able to log in to Telnet and send mail as another user.

I've just checked the Headers and it seems I was a bit confused - all it seems to be doing is spoofing the 'from' email address.

It records my IP as the originating IP - which is fine - I understand that.

How do I 'authenticate' as a user via Telnet?

Also, what exactly does the EHLO/HELO command do? I can identify myself there as whatever I want?